Documentation site and CI workflows

An mdBook under docs/book: a user guide (getting started, UI tour, sketching, features,
parameters, components and joints, sheet metal, plastic, files, shortcuts, MCP) and a developer
guide (CONTRIBUTING, architecture, command engine, kernel boundary, persistent naming, test
harnesses, and the existing docs/*.md pages). `cargo xtask book` generates the command reference
from `solvecraft-cli commands` and renders the screenshots with `solvecraft-cli snapshot`, then
runs mdbook. .github/workflows/pages.yml publishes the book at /solvecraft/ and the web app at
/solvecraft/app/ (skipped while the repository is private). .github/workflows/ci.yml runs
`cargo xtask ci` on pull requests. README and Help → Documentation link to the book.

Fixes #28
Fixes #8

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/guide/plastic.md

@@ -0,0 +1,13 @@
+# Plastic parts
+
+The PLASTIC tab has a small enclosure subset:
+
+- **Boss** (`plastic.boss`): screw bosses with a hole, fillets and optional ribs.
+- **Lip / Groove** (`plastic.lip`): the mating lip and groove along a shell's opening.
+- **Snap Fit** (`plastic.snap_fit`): a cantilever snap hook and its catch.
+- **Rest** (`plastic.rest`): a pad for a part to rest on.
+- **Plastic Rules** (`plastic.manage_rules`, `plastic.assign_rule`): wall thickness, draft and
+  clearance per material, used as the features' defaults.
+
+Rib and web (on the SOLID tab) complete the set. These features are checked against
+hand-computed geometry rather than a Fusion oracle.